An infinite dimensional umbral calculus Dmitri Finkelshtein Department of Mathematics, Swansea University, Singleton Park, Swansea SA2 8PP, U.K.; e-mail: [email protected] Yuri Kondratiev Fakult¨atf¨urMathematik, Universit¨atBielefeld, 33615 Bielefeld, Germany; e-mail: [email protected] Eugene Lytvynov Department of Mathematics, Swansea University, Singleton Park, Swansea SA2 8PP, U.K.; e-mail: [email protected] Maria Jo˜aoOliveira Departamento de Ciˆenciase Tecnologia, Universidade Aberta, 1269-001 Lisbon, Por- tugal; CMAF-CIO, University of Lisbon, 1749-016 Lisbon, Portugal; e-mail: [email protected]
Abstract
The aim of this paper is to develop foundations of umbral calculus on the space D0 of distri- d 0 butions on R , which leads to a general theory of Sheffer polynomial sequences on D . We define a sequence of monic polynomials on D0, a polynomial sequence of binomial type, and a Sheffer sequence. We present equivalent conditions for a sequence of monic polynomials on D0 to be of binomial type or a Sheffer sequence, respectively. We also construct a lift- ing of a sequence of monic polynomials on R of binomial type to a polynomial sequence of 0 0 binomial type on D , and a lifting of a Sheffer sequence on R to a Sheffer sequence on D . Examples of lifted polynomial sequences include the falling and rising factorials on D0, Abel, Hermite, Charlier, and Laguerre polynomials on D0. Some of these polynomials have already appeared in different branches of infinite dimensional (stochastic) analysis and played there a fundamental role. arXiv:1701.04326v2 [math.FA] 5 Mar 2018 Keywords: Generating function; polynomial sequence on D0; polynomial sequence of binomial type on D0; Sheffer sequence on D0; shift-invariance; umbral calculus on D0. 2010 MSC. Primary: 05A40, 46E50. Secondary: 60H40, 60G55.
1 Introduction
In its modern form, umbral calculus is a study of shift-invariant linear operators acting on polynomials, their associated polynomial sequences of binomial type, and Sheffer sequences (including Appell sequences). We refer to the seminal papers [29, 38, 39],
1 see also the monographs [24, 37]. Umbral calculus has applications in combinatorics, theory of special functions, approximation theory, probability and statistics, topology, and physics, see e.g. the survey paper [12] for a long list of references. Many extensions of umbral calculus to the case of polynomials of several, or even infinitely many variables were discussed e.g. in [5,10,14,28,32,35,36,41,42], for a longer list of such papers see the introduction to [13]. Appell and Sheffer sequences of poly- nomials of several noncommutative variables arising in the context of free probability, Boolean probability, and conditionally free probability were discussed in [2–4], see also the references therein. The paper [13] was a pioneering (and seemingly unique) work in which elements of basis-free umbral calculus were developed on an infinite dimensional space, more precisely, on a real separable Hilbert space H. This paper discussed, in particular, shift- invariant linear operators acting on the space of polynomials on H, Appell sequences, and examples of polynomial sequences of binomial type. In fact, examples of Sheffer sequences, i.e., polynomial sequences with generating function of a certain exponential type, have appeared in infinite dimensional analysis on numerous occasions. Some of these polynomial sequences are orthogonal with respect to a given probability measure on an infinite dimensional space, while others are related to analytical structures on such spaces. Typically, these polynomials are either defined on a co-nuclear space Φ0 (i.e, the dual of a nuclear space Φ), or on an appropriate subset of Φ0. Furthermore, in majority of examples, the nuclear space Φ consists of (smooth) functions on an underlying space X. For simplicity, we choose to work in this paper with the Gel’fand triple
2 d 0 0 Φ = D ⊂ L (R , dx) ⊂ D = Φ .
Here D is the nuclear space of smooth compactly supported functions on Rd, d ∈ N, and D0 is the dual space of D, where the dual pairing between D0 and D is obtained by continuously extending the inner product in L2(Rd, dx). Let us mention several known examples of Sheffer sequences on D0 or its subsets:
(i) In infinite dimensional Gaussian analysis, also called white noise analysis, Her- mite polynomial sequences on D0 (or rather on S0 ⊂ D0, the Schwartz space of tempered distributions) appear as polynomials orthogonal with respect to Gaus- sian white noise measure, see e.g. [6,15,16,31]. (ii) Charlier polynomial sequences on the configuration space of counting Radon mea- sures on Rd,Γ ⊂ D0, appear as polynomials orthogonal with respect to Poisson point process on Rd, see [17,19,22]. (iii) Laguerre polynomial sequences on the cone of discrete Radon measures on Rd, K ⊂ D0, appear as polynomials orthogonal with respect to the gamma random measure, see [21,22].
2 (iv) Meixner polynomial sequences on D0 appear as polynomials orthogonal with re- spect to the Meixner white noise measure, see [25,26].
(v) Special polynomials on the configuration space Γ ⊂ D0 are used to construct the K-transform, see e.g. [7,18,20]. Recall that the K-transform determines the duality between point processes on Rd and their correlation measures. These polynomials will be identified in this paper as the infinite dimensional analog of the falling factorials (a special case of the Newton polynomials).
(vi) Polynomial sequences on D0 with generating function of a certain exponential type are used in biorthogonal analysis related to general measures on D0, see [1,23].
Note, however, that even the very notion of a general polynomial sequence on an infinite dimensional space has never been discussed! The classical umbral calculus on the real line gives a general theory of Sheffer sequences and related (umbral) operators. So our aim in this paper is to develop foundations of umbral calculus on the space D0, which will eventually lead to a general theory of Sheffer sequences on D0 and their umbral operators. In fact, at a structural level, many results of this paper will have remarkable similarities to the classical setting of polynomials on R. For example, the form of the generating function of a Sheffer sequence on D0 will be similar to the generating function of a Sheffer sequence on R: the constants appearing in the latter function are replaced in the former function by appropriate linear continuous operators. There is a principal point in our approach that we would like to stress. The paper [13] deals with polynomials on a general Hilbert space H, while the monograph [6] develops Gaussian analysis on a general co-nuclear space Φ0, without the assumption that Φ0 consists of generalized functions on Rd (or on a general underlying space). In fact, we will discuss in Remark 7.5 below that the case of the infinite dimensional Hermite polynomials is, in a sense, exceptional and does not require from the co-nuclear space Φ0 any special structure. In all other cases, the choice Φ0 = D0 is crucial. Having said this, let us note that our ansatz can still be applied to a rather general co-nuclear space of generalized functions over a topological space X, equipped with a reference measure. We also stress that the topological aspects of the spaces D, D0, and their sym- metric tensor powers are crucial for us since all the linear operators (appearing as ‘coefficients’ in our theory) are continuous in the respective topologies. Furthermore, this assumption of continuity is principal and cannot be omitted. The origins of the classical umbral calculus are in combinatorics. So, by analogy, one can think of umbral calculus on D0 as a kind of spatial combinatorics. To give the reader P∞ a better feeling of this, let us consider the following example. Let γ = i=1 δxi ∈ Γ be a configuration. Here δxi denotes the Dirac measure with mass at xi. We will construct (the kernel of) the falling factorial, denoted by (γ)n, as a function from Γ to
3 D0 n. (Here and below denotes the symmetric tensor product.) This will allow us γ 1 to define ‘γ choose n’ by n := n! (γ)n. And we will get the following explicit formula which supports this term:
γ X = δx δx · · · δx , (1.1) n i1 i2 in {i1,...,in}⊂N
i.e., the sum is obtained by choosing all possible n-point subsets from the (locally finite)
set {xi}i∈N. The latter set can be obviously identified with the configuration γ. The paper is organized as follows. In Section 2 we discuss preliminaries. In partic- 0 ular, we recall the construction of a general Gel’fand triple Φ ⊂ H0 ⊂ Φ , where Φ is a nuclear space and Φ0 is the dual of Φ (a co-nuclear space) with respect to the center 0 0 Hilbert space H0. We consider the space P(Φ ) of polynomials on Φ and equip it with a nuclear space topology. Its dual space, denoted by F(Φ0), has a natural (commutative) algebraic structure with respect to the symmetric tensor product. We also define a family of shift operators, (E(ζ))ζ∈Φ0 , and the space of shift-invariant continuous linear operators on P(Φ0), denoted by S(P(Φ0)). In Section 3, we give the definitions of a polynomial sequence on Φ0, a monic polynomial sequence on Φ0, and a monic polynomial sequence on Φ0 of binomial type. Starting from Section 4, we choose the Gel’fand triple as D ⊂ L2(Rd, dx) ⊂ D0. The main result of this section is Theorem 4.1, which gives three equivalent conditions for a monic polynomial sequence to be of binomial type. The first equivalent condition is that the corresponding lowering operators are shift-invariant. The second condition gives a representation of each lowering operator through directional derivatives in directions d of delta functions, δx (x ∈ R ). The third condition gives the form of the generating function of a polynomial sequence of binomial type. To prove Theorem 4.1, we derive two essential results. The first one is an operator expansion theorem (Theorem 4.7), which gives a description of any shift-invariant 0 operator T ∈ S(P(D )) in terms of the lowering operators in directions δx. The second result is an isomorphism theorem (Theorem 4.9): we construct a bijection J between the spaces S(P(D0)) and F(D0) such that, under J, the product of any shift-invariant operators goes over into the symmetric tensor product of their images. This implies, in particular, that any two shift-invariant operators commute. Next, we define a family of delta operators on D0 and prove that, for each such family, there exists a unique monic polynomial sequence of binomial type for which these delta operators are the lowering operators. In Section 5, we identify a procedure of the lifting of a polynomial sequence of binomial type on R to a polynomial sequence of binomial type on D0. This becomes possible due to the structural similarities between the one-dimensional and infinite- dimensional theories. Using this procedure, we identify, on D0, the falling factorials, the rising factorials, the Abel polynomials, and the Laguerre polynomials of binomial
4 type. We stress that the polynomial sequences lifted from R to D0 form a subset of a (much larger) set of all polynomial sequences of binomial type on D0. In Section 6, we define a Sheffer sequence on D0 as a monic polynomial sequence on D0 whose lowering operators are delta operators. Thus, to every Sheffer sequence, there corresponds a (unique) polynomial sequence of binomial type. In particular, if the corresponding binomial sequence is just the set of monomials (i.e., their delta operators are differential operators), we call such a Sheffer sequence an Appell sequence. The main result of this section, Theorem 6.2, gives several equivalent conditions for a monic polynomial sequence to be a Sheffer sequence. In particular, we find the generating function of a Sheffer sequence on D0. In Section 7, we extend the procedure of the lifting described in Section 5 to Sheffer sequences. Thus, for each Sheffer sequence on R, we define a Sheffer sequence on D0. Using this procedure, we recover, in particular, the Hermite polynomials, the Charlier polynomials, and the orthogonal Laguerre polynomials on D0. Finally, in Appendix, we discuss several properties of formal tensor power series. From the technical point of view, the similarities between the infinite dimensional and the classical settings open new perspectives in infinite dimensional analysis. Due to the special character of this approach, namely, through definition of umbral operators on P(D0) and umbral composition of polynomials on D0, further developments and applications in infinite dimensional analysis are subject of forthcoming publications. Let us also mention the open problem of (at least partial) characterization of Sheffer sequences that are orthogonal with respect to a certain probability measure on D0. In the one-dimensional case, such a characterization is due to Meixner [27]. For multi- dimensional extensions of this result, see [11,33,34] and the references therein.
2 Preliminaries
2.1 Nuclear and co-nuclear spaces Let us first recall the definition of a nuclear space, for details see e.g. [8, Chapter 14, Section 2.2]. Consider a family of real separable Hilbert spaces (Hτ )τ∈T , where T is T an arbitrary indexing set. Assume that the set Φ := τ∈T Hτ is dense in each Hilbert space Hτ and the family (Hτ )τ∈T is directed by embedding, i.e., for any τ1, τ2 ∈ T there exists a τ3 ∈ T such that Hτ3 ⊂ Hτ1 and Hτ3 ⊂ Hτ2 and both embeddings are continuous. We introduce in Φ the projective limit topology of the Hτ spaces:
Φ = proj lim Hτ . τ∈T
By definition, the sets {ϕ ∈ Φ | kϕ − ψkHτ < ε} with ψ ∈ Φ, τ ∈ T , and ε > 0 form a system of base neighborhoods in this topology. Here k · kHτ denotes the norm in Hτ .
5 Assume that, for each τ1 ∈ T , there exists a τ2 ∈ T such that Hτ2 ⊂ Hτ1 , and the
operator of embedding of Hτ2 into Hτ1 is of the Hilbert–Schmidt class. Then the linear topological space Φ is called nuclear. Next, let us assume that, for some τ0 ∈ T , each Hilbert space Hτ with τ ∈ T is continuously embedded into H0 := Hτ0 . We will call H0 the center space. 0 Let Φ denote the dual space of Φ with respect to the center space H0, i.e., the dual pairing between Φ0 and Φ is obtained by continuously extending the inner product in 0 H0, see e.g. [8, Chapter 14, Section 2.3]. The space Φ is often called co-nuclear. 0 S By the Schwartz theorem (e.g. [8, Chapter 14, Theorem 2.1]), Φ = τ∈T H−τ , where H−τ denotes the dual space of Hτ with respect to the center space H0. We endow Φ0 with the Mackey topology—the strongest topology in Φ0 consistent with the duality between Φ and Φ0 (i.e., the set of continuous linear functionals on Φ0 coincides with Φ). The Mackey topology in Φ0 coincides with the topology of the inductive limit of the H−τ spaces, see e.g. [40, Chapter IV, Proposition 4.4] or [6, Chapter 1, Section 1]. Thus, we obtain the Gel’fand triple (also called the standard triple)
0 Φ = proj lim Hτ ⊂ H0 ⊂ ind lim H−τ = Φ . (2.1) τ∈T τ∈T Let X and Y be linear topological spaces that are locally convex and Hausdorff. (Both Φ and Φ0 are such spaces.) We denote by L(X,Y ) the space of continuous linear operators acting from X into Y . We will also denote L(X) := L(X,X). We denote by X0 and Y 0 the dual space of X and Y , respectively. We endow X0 with the Mackey topology with respect to the duality between X and X0. We similarly endow Y 0 with the Mackey topology. Each operator A ∈ L(X,Y ) has the adjoint operator A∗ ∈ L(Y 0,X0) (also called the transpose of A or the dual of A), see e.g. [30, Theorem 8.11.3]. Remark 2.1. Note that, since we chose the Mackey topology on Φ0, for an operator A ∈ L(Φ0), we have A∗ ∈ L(Φ). This fact will be used throughout the paper.
Proposition 2.2. Consider the Gel’fand triple (2.1). Let A :Φ → Φ and B :Φ0 → Φ0 be linear operators. (i) We have A ∈ L(Φ) if and only if, for each τ1 ∈ T , there exists a τ2 ∈ T such ˆ that the operator A can be extended by continuity to an operator A ∈ L(Hτ2 , Hτ1 ). 0 (ii) We have B ∈ L(Φ ) if and only if, for each τ1 ∈ T , there exists a τ2 ∈ T such ˆ ˆ that the operator B := B H−τ1 takes on values in H−τ2 and B ∈ L(H−τ1 , H−τ2 ). Remark 2.3. Proposition 2.2 admits a sraightforward generalization to the case of two 0 0 Gel’fand triples, Φ ⊂ H0 ⊂ Φ and Ψ ⊂ G0 ⊂ Ψ , and linear operators A :Φ → Ψ and B :Φ0 → Ψ0. Remark 2.4. Part (ii) of Proposition 2.2 is related to the universal property of an inductive limit, which states that any linear operator from an inductive limit of a
6 family of locally convex spaces to another locally convex space is continuous if and only if the restriction of the operator to any member of the family is continuous, see e.g. [9, II.29]. Proof of Proposition 2.2. (i) By the definition of the topology in Φ, the linear operator A :Φ → Φ is continuous if and only if, for any τ1 ∈ T and ε1 > 0, there exist τ2 ∈ T and ε2 > 0 such that the pre-image of the set {ϕ ∈ Φ | kϕk < ε } Hτ1 1 contains the set {ϕ ∈ Φ | kϕk < ε }. Hτ2 2 But this implies the statement. (ii) Assume B ∈ L(Φ0). Then, by Remark 2.1, we have B∗ ∈ L(Φ). Hence, for each ∗ τ1 ∈ T , there exists a τ2 ∈ T such that the operator B can be extended by continuity ˆ∗ ˆ∗ ˆ to an operator B ∈ L(Hτ2 , Hτ1 ). But the adjoint of the operator B is B := B H−τ1 . ˆ Hence B ∈ L(H−τ1 , H−τ2 ). Conversely, assume that, for each τ1 ∈ T , there exists a τ2 ∈ T such that the ˆ ˆ operator B := B H−τ1 takes on values in H−τ2 and B ∈ L(H−τ1 , H−τ2 ). Therefore, ˆ∗ ˆ∗ B ∈ L(Hτ2 , Hτ1 ). Denote A := B Φ. As easily seen, the definition of the operator A does not depend on the choice of τ1, τ2 ∈ T . Hence, A :Φ → Φ, and by part (i) we conclude that A ∈ L(Φ). But B = A∗ and hence B ∈ L(Φ0). In what follows, ⊗ will denote the tensor product. In particular, for a real separable ⊗n Hilbert space H, H denotes the nth tensor power of H. We will denote by Symn ∈ L(H⊗n) the symmetrization operator, i.e., the orthogonal projection satisfying 1 X Sym f ⊗ f ⊗ · · · ⊗ f = f ⊗ f ⊗ · · · ⊗ f (2.2) n 1 2 n n! σ(1) σ(2) σ(n) σ∈S(n) for f1, f2, . . . , fn ∈ H. Here S(n) denotes the symmetric group acting on {1, . . . , n}. We will denote the symmetric tensor product by . In particular,
f1 f2 · · · fn := Symn f1 ⊗ f2 ⊗ · · · ⊗ fn, f1, f2, . . . , fn ∈ H,
n ⊗n and H := Symn H is the nth symmetric tensor power of H. Note that, for each f ∈ H, we have f n = f ⊗n. Starting with Gel’fand triple (2.1), one constructs its nth symmetric tensor power as follows: n n n n 0 n Φ := proj lim Hτ ⊂ H0 ⊂ ind lim H−τ =: Φ , τ∈T τ∈T see e.g. [6, Section 2.1] for details. In particular, Φ n is a nuclear space and Φ0 n is its n 0 0 0 0 dual with respect to the center space H0 . We will also denote Φ = H0 = Φ := R. The dual pairing between F (n) ∈ Φ0 n and g(n) ∈ Φ n will be denoted by hF (n), g(n)i.
7 Remark 2.5. Consider the set {ξ⊗n | ξ ∈ Φ}. By the polarization identity, the linear n span of this set is dense in every space Hτ , τ ∈ T . The following lemma will be very important for our considerations.
Lemma 2.6. (i) Let F (n),G(n) ∈ Φ0 n be such that
hF (n), ξ⊗ni = hG(n), ξ⊗ni for all ξ ∈ Φ,
then F (n) = G(n). (ii) Let Φ and Ψ be nuclear spaces and let A, B ∈ L(Φ n, Ψ). Assume that
Aξ⊗n = Bξ⊗n for all ξ ∈ Φ.
Then A = B.
Proof. Statement (i) follows from Remark 2.5, statement (ii) follows from Proposi- tion 2.2, (i) and Remarks 2.3 and 2.5.
2.2 Polynomials on a co-nuclear space Below we fix the Gel’fand triple (2.1). Definition 2.7. A function P :Φ0 → R is called a polynomial on Φ0 if
n X P (ω) = hω⊗k, f (k)i, ω ∈ Φ0, (2.3) k=0
(k) k ⊗0 (n) where f ∈ Φ , k = 0, 1, . . . , n, n ∈ N0 := {0, 1, 2,... }, and ω := 1. If f 6= 0, one says that the polynomial P is of degree n. We denote by P(Φ0) the set of all polynomials on Φ0. Remark 2.8. For each P ∈ P(Φ0), its representation in form (2.3) is evidently unique. (k) k (n) n For any f ∈ Φ and g ∈ Φ , k, n ∈ N0, we have hω⊗k, f (k)ihω⊗n, g(n)i = hω⊗(k+n), f (k) g(n)i, ω ∈ Φ0. (2.4)
Hence P(Φ0) is an algebra under point-wise multiplication of polynomials on Φ0. 0 We will now define a topology on P(Φ ). Let Ffin(Φ) denote the topological direct n sum of the nuclear spaces Φ , n ∈ N0. Hence, Ffin(Φ) is a nuclear space, see e.g. [6, Section 5.1]. This space consists of all finite sequences f = (f (0), f (1), . . . , f (n), 0, 0,... ), (k) k where f ∈ Φ , k = 0, 1, . . . , n, n ∈ N0. The convergence in Ffin(Φ) means the uniform finiteness of non-zero elements and the coordinate-wise convergence in each Φ k.
8 Remark 2.9. Below we will often identify f (n) ∈ Φ n with
(n) (0,..., 0, f , 0, 0,... ) ∈ Ffin(Φ).
0 We define a natural bijective mapping I : Ffin(Φ) → P(Φ ) by n X (If)(ω) := hω⊗k, f (k)i, (2.5) k=0
(0) (1) (n) for f = (f , f , . . . , f , 0, 0,... ) ∈ Ffin(Φ). We define a nuclear space topology on 0 P(Φ ) as the image of the topology on Ffin(Φ) under the mapping I. The space Ffin(Φ) may be endowed with the structure of an algebra with respect to the symmetric tensor product
k !∞ X f g = f (i) g(k−i) , (2.6) i=0 k=0 (k) ∞ (k) ∞ where f = (f )k=0, g = (g )k=0 ∈ Ffin(Φ). The unit element of this (commutative) algebra is the vacuum vector Ω := (1, 0, 0 ...). By (2.4)–(2.6), the bijective mapping I provides an isomorphism between the alge- 0 bras Ffin(Φ) and P(Φ ), namely, for any f, g ∈ Ffin(Φ), I(f g)(ω) = (If)(ω)(Ig)(ω), ω ∈ Φ0. Let ∞ Y F(Φ0) := Φ0 k k=0 denote the topological product of the spaces Φ0 k. The space F(Φ0) consists of all (k) ∞ (k) 0 k sequences F = (F )k=0, where F ∈ Φ , k ∈ N0. Note that the convergence in this space means the coordinate-wise convergence in each space Φ0 k. (k) ∞ 0 Each element F = (F )k=0 ∈ F(Φ ) determines a continuous linear functional on Ffin(Φ) by ∞ X (k) (k) (k) ∞ hF, fi := hF , f i, f = (f )k=0 ∈ Ffin(Φ) (2.7) k=0 0 (note that the sum in (2.7) is, in fact, finite). The dual of Ffin(Φ) is equal to F(Φ ), and the topology on F(Φ0) coincides with the Mackey topology on F(Φ0) that is consistent 0 with the duality between Ffin(Φ) and F(Φ ), see e.g. [6]. In view of the definition of the topology on P(Φ0), we may also think of F(Φ0) as the dual space of P(Φ0). Similarly to (2.6), one can introduce the symmetric tensor product on F(Φ0):
k !∞ X F G = F (i) G(k−i) , (2.8) i=0 k=0
9 (k) ∞ (k) ∞ 0 where F = (F )k=0,G = (G )k=0 ∈ F(Φ ). The unit element of this algebra is again Ω = (1, 0, 0,...). We will now discuss another realization of the space F(Φ0). We denote by S(R, R) P∞ n the vector space of formal series R(t) = n=0 rnt in powers of t ∈ R, where rn ∈ R for n ∈ N0. The S(R, R) is an algebra under the product of formal power series. Similarly to S(R, R), we give the following (n) ∞ 0 Definition 2.10. Each (F )n=0 ∈ F(Φ ) identifies a ‘real-valued’ formal series P∞ (n) ⊗n n=0hF , ξ i in tensor powers of ξ ∈ Φ. We denote by S(Φ, R) the vector space of such formal series with natural operations. We define a product on S(Φ, R) by
∞ ! ∞ ! ∞ * n + X X X X hF (n), ξ⊗ni hG(n), ξ⊗ni = F (i) G(n−i), ξ⊗n , (2.9) n=0 n=0 n=0 i=0
(n) ∞ (n) ∞ 0 where (F )n=0, (G )n=0 ∈ F(Φ ). (n) ∞ (n) ∞ 0 Remark 2.11. Assume that, for some (F )n=0, (G )n=0 ∈ F(Φ ) and ξ ∈ Φ, both P∞ (n) ⊗n P∞ (n) ⊗n series n=0hF , ξ i and n=0hG , ξ i converge absolutely. Then also the series on the right hand side of (2.9) converges absolutely and (2.9) holds as an equality of two real numbers. (n) ∞ 0 Remark 2.12. Let t ∈ R and ξ ∈ Φ. Then, tξ ∈ Φ and for (F )n=0 ∈ F(Φ ),
∞ ∞ X X hF (n), (tξ)⊗ni = tnhF (n), ξ⊗ni, (2.10) n=0 n=0 the expression on the right hand side of equality (2.10) being the formal power series in t that has coefficient hF (n), ξ⊗ni by tn. According to the definition of S(Φ, R), there exists a natural bijective mapping I : F(Φ0) → S(Φ, R) given by
∞ X (n) ⊗n (n) ∞ 0 (IF )(ξ) := hF , ξ i,F = (F )n=0 ∈ F(Φ ), ξ ∈ Φ. (2.11) n=0
The mapping I provides an isomorphism between the algebras F(Φ0) and S(Φ, R), namely, for any F,G ∈ F(Φ0),